[keyserver] Extract getBackupInfos

Summary:
We're going to want to reuse some of this code for ENG-1234, so I extracted it.

Depends on D4262

Test Plan: I copy pasted the new backups.js file into prod and added a console.log of the result of getBackupInfos. I made sure it was in chrono order and had everything right

Reviewers: atul, yayabosh

Reviewed By: atul

Subscribers: palys-swm, Adrian

Differential Revision: https://phab.comm.dev/D4263